1. Field of the Invention
The present invention relates to a method of continuously manufacturing a metallic bent pipe having straight pipe portions at both ends of a bent pipe portion, which have been subjected to the same heat treatment as that for the bent pipe portion.
2. Description of the Prior Art
It is desirable to employ what is called a winged bent pipe, which has straight pipe portions at both ends of a bent pipe portion which have been subjected to the same heat treatment as that for the bent pipe portion, as a metallic pipe required to have a high strength and a high toughness such as a pipe employed for a bent portion of a pipeline laid in a sea area under severe natural conditions or a northernmost cold zone or a pipe for a pipeline required to cope with a transporting pressure having recently been raised in order to increase the quantity of transportation. The reason therefor is that in case of carrying out edge preparation or butt welding, the operation is extremely easy, since not the bent pipe portion but the straight pipe portions are subjected to such a processing to conduct the operation. Moreover, when a welding defect occurs at a welded joint after welding, the defective portion is generally cut off and welding is carried out again. Also in such a case, a straight pipe portion is subjected to such operations; hence, the operations are extremely easy.
Further, in the piping used for a nuclear reactor or other piping, a stainless steel pipe is generally subjected to heat treatment such as solid solution treatment in order to improve reliability. In such a case, if the heat treatment can be applied to not only a bent pipe portion but also straight pipe portions leading thereto continuously in one step, any discontinuous heat treatment is eliminated, so that it is possible to further improve reliability.
Hitherto, it has been known that heat treatment permits a steel pipe to be higher in strength and toughness, resulting in a steel pipe having a high quality level. As the method of heat-treating a straight steel pipe over its overall length, such a method has been known that a heating means such as a high-frequency induction heater is disposed on the longitudinal front end portion of a steel pipe, and while the pipe is being heated in an annular zone by the heating means, the heater is relatively moved toward the rear end portion of the pipe, thereby to properly cool the heated portion. On the other hand, as the method of bending a steel pipe such as mentioned above, such a method has been known that a portion of the pipe to be bent is annularly and locally heated by the above-mentioned heating means and cooled immediately after the heating, and while the heating zone is being relatively moved in the longitudinal direction of the pipe, the pipe is transformed by applying a bending moment thereto. The bent pipe portion bent by this method is subjected to heat treatment simultaneously with the bending processing.
Thus, a metallic bent pipe having straight pipe portions at both ends of a bent pipe portion which have been subjected to the same heat treatment as that for the bent pipe portion is conventionally manufactured as follows. After a bent pipe portion is formed by bending processing according to a method such as described above, straight pipe portions are subjected to heat treatment by a method such as described above, or all the pipe portions except for a portion to be a bent pipe portion are previously heat-treated by the above-mentioned method and then the straight pipe portion to be a bent pipe portion is bent by the above-mentioned method. These conventional methods, however, disadvantageously require two steps, i.e., the bending processing and the heat treatment for the straight pipe portions. Moreover, it is extremely difficult for these conventional methods to uniform the heat-treatment conditions of the joining portions between the bent pipe portion and the straight pipe portions or the conditions of the advancing heat-treated portion and the subsequent heat-treated portion.
Accordingly, in order to manufacture such a metallic bent pipe having straight pipe portions at both ends of a bent pipe portion which have been subjected to the same heat treatment as that for the bent pipe portion, it is desirable to apply heat treatment to the straight pipe portions while subjecting the pipe to bending processing. However, although the straight pipe portions can be heat-treated simply by fixing either the pipe or the heating means and moving the other and then cooling the heated portion, the bending processing generates an extremely large bending moment to the pipe, causing a large load to be applied to a clamp support for the pipe. Therefore, since the support for the pipe is constructed as a rigid body capable of sufficiently bearing this load and has a certain size with respect to the longitudinal direction of the pipe, it is necessary to remove and remount the pipe clamp to shift the heat-treatment zone the straight pipe portions to the bending portion or from the bending portion to the straight pipe portions. Accordingly, it is essential to devise such that the variation in load in accordance with the removal and remounting of the pipe clamp will not adversely affect the bending processing or the heat treatment. Therefore, there are still many problems to be solved in order to continuously carry out the manufacture of a metallic bent pipe such as mentioned above.
However, if such a method is developed which is capable of continuously manufacturing a metallic bent pipe having straight pipe portions at both ends of a bent pipe portion, which have been subjected to the same heat treatment as that for the bent pipe portion, the method is exceedingly useful from the industrial viewpoint.